This invention relates to a method which can refine scrap silicon rapidly and in large quantities using an electron beam to obtain high-purity silicon suitable for solar cells, semiconductors, and similar devices.
In order to increase the rate of utilization of silicon raw materials, it is desired to reutilize scrap which is produced during the manufacture of silicon wafers from silicon ingots (referred to below as “scrap silicon”). However, scrap silicon contains impurities, so it is necessary to increase its purity before it can be reused.
Japanese Published Unexamined Patent Application Hei 10-245216 discloses that the yield of silicon can be increased by recovering scrap silicon which is produced during the manufacture of silicon for solar cells and returning the recovered scrap silicon to a refining step.
However, that document merely discloses a technique for improving the yield of silicon in the manufacture of silicon for solar cells. Lumps of scrap silicon from different sources may contain a variety of different impurity elements, such as boron, phosphorous, arsenic, and antimony. Depending upon the type of impurity element, some lumps cannot be refined using an electron beam, while there are lumps for which refining with an electron beam is possible but is nevertheless impractical from the standpoints of refining time and refining costs. In addition, when lumps of scrap silicon containing different impurity elements are mixed together during the melting stage of a refining process, high-purity silicon is not obtained. Furthermore, if scrap silicon contains impurity elements which cannot be removed adequately or at all by melting, the impurity elements end up remaining in the silicon after refining, and high-purity silicon is not obtained.